*1.2. AMC Type Classification for Cutting Tools*

The options for wear and the reasons for cutting tool efficiency loss when varying within a wide range of their operating conditions were considered above. All AMCs for cutting tool purposes must withstand high contact pressures and temperatures to function directly in the cutting zone or near it. Authoritative research groups develop and apply in practice various technological approaches considering these specific conditions, thanks to which the working sections of the cutting tool can adapt to the external environment effects. The authors of this work propose an array of available scientific and technological approaches in the field of AMCs for cutting tools to classify them into three main groups (Figure 1 shows a variant of the graphical visualization of the proposed classification):


**Figure 1.** Classification of AMC types to improve the performance of the cutting tool.

#### *1.3. Materials and Coatings with Active Control Function*

The group of materials and coatings with active control function should include AMCs, which in their essence play the role of sensors that, when exposed to external influences, recognize (detect) critical changes during the cutting tool operation and transmit the corresponding signals wirelessly to the control system of the metal-cutting machine. The system carries out a control (adaptive) effect on cutting according to the built-in algorithms, depending on the information received and the technological problems to be solved. For example, it adjusts the cutting speed, feed, or depth to reduce the likelihood of brittle fracture of the cutting part or reduce tool excessive wear. In emergency cases, it stops machining, thus realizing the active control principle.

When implementing this approach, coatings can be applied to the cutting tool (for example, thin-film thermocouples), or heat-sensitive and elastic-sensitive inserts can be used in the tool design, located near the cutting zone, which demonstrate deformations or other physical parameters characterizing the machining and cutting tool state [29–31]. It is advisable to use these AMCs in the manufacture of the most critical products in the aerospace, nuclear, and other industries when there is a technological task of manufacturing either an extended (large) part or a complex-shaped part for which a cutting tool failure during machining will inevitably lead to a failure of expensive products. Another application area is special alloys, difficult-to-machine composites, and machining of other materials when there is a high probability of unexpected cutting tool failure.

There has been research on online monitoring of cutting temperature during continuous turning of titanium and nickel alloys using interchangeable triangular and tetrahedral cutting inserts made of WC–Co hard alloy with built-in chromel–alumel thermocouple sensors. Microgrooves with a depth of about 100 μm are etched on the flank face using laser action to accommodate thin-film thermocouples on carbide inserts near the cutting edge, and the surface of the cutting tool is covered with a thick insulating layer [29]. The sensors can be staggered concerning the direction of the chip flow to increase the measurement accuracy. Detailed technological principles and options for thin-film thermocouples forming on the tool surface are described in [29–32]. Well-known studies have demonstrated the possibility of high-precision measurement in cutting temperature values in the range of up to 1000 ◦C. In this case, the natural wear of the cutting edge of the carbide inserts does not affect the performance of the sensors and the reliability of the received diagnostic data. There are experimental data on the efficiency of using the above-described cutting platesensors during operation under intermittent cutting with difficult-to-machine material conditions [30]. Under such conditions, sensors based on chromel–alumel thermocouples demonstrate high sensitivity and low inertia. There has been some research in which thin-film thermocouples were embedded in polycrystalline cubic boron nitride cutting inserts by diffusion bonding to control turning of very hard materials [31].

Additionally, it should be noted that it is promising to use sensors based on flexible piezoelectric polymer films to design structures and for cutting tool production. This gives an excellent response under mechanical stress action and allows for assessing the components of the cutting force according to the degree of their deformation, which is an informative diagnostic (troubleshooting) sign of wear and tear of tool surfaces and the state of the entire metalworking system as a whole [33,34]. For these purposes, it is most convenient to use PVDF (polyvinylidene fluoride) sensors (film) with a multilayer structure consisting of a PVDF film sandwiched between two electrodes and protective coatings [35]. The locations for installing the sensors are variable, but they should be as close as possible to the processing area (in this case, it is necessary to consider that the working temperature of the PVDF film is 700 ◦C). They are installed in seats under each cutting plate in the tool body as a rule (in the case of using a multiblade assembly tool) or in special recesses made in the tool holder, into which the cutting plate abuts with the working surfaces and where elastic deformations arise under the action of cutting forces. The timely response of the control system to the signals transmitted by the sensors allows the cutting tool to be adapted to changed conditions, thereby preserving its performance for a longer time.

### *1.4. Self-Organizing Materials and Coatings*

This group should include AMCs capable of providing the structural self-organization of the surface layer of the cutting tool during contact interaction with the environment and workpiece being machined under conditions of heat–power loads typical for cutting processes due to secondary structure (phase) formation. It is the most extensive AMC group in terms of implementation options and research depth, with which many authoritative world-class teams work [15,36–45]. A classic example of secondary structures is those formed at elevated cutting temperatures in conditions of interaction with the natural environment (air) or artificially introduced external media of oxide and sometimes nitride compounds of metals of IV–VI groups of the periodic table. Secondary structures have thermal stability and improved lubricity in the tribocontact zone, significantly reduce the intensity of frictional interaction, and increase the wear resistance of the tool contact pads.

A schematic diagram demonstrating the stages of secondary structures forming during physicochemical processes in the contact zone in cutting materials is proposed by the authors in Figure 2. At the initial stage (stage 1, Figure 2), when the cutting wedge separates the chips from the workpiece, chemically clean surfaces (often called juvenile) are formed on the tool and the workpiece, on which there are no oxides and adsorbed films. "Freshly" formed surfaces are of a very active energetic and electronic state, and they are ready to interact with the components of the external environment (stage 2, Figure 2), which can be characterized by various sorption processes, primarily physical and chemical adsorption [46–48]. The energy of the broken molecular bonds of the juvenile surface is such that the external environment molecules can undergo destruction and decay into atoms, ions, and radicals. These formed particles are also chemically active and can enter into chemical interaction with freshly formed metal surfaces. At the next moment, the formation of new chemical compounds (secondary structures) is observed in the "tool-processed material" contact zone due to reactions between chemically pure

surfaces and components of the external environment, that is, the processes of ion and chemisorption (stage 3, Figure 2). The physicochemical processes in the contact zone with protective (lubricating) films and secondary structures forming at the interface with thermal stability and improved lubricity passivate the adhesion and frictional interaction between chemically pure surfaces. The secondary structures represent a stable zone with an increased internal energy level from the thermodynamics point [49–51]. The transformation of the conditions for contacting the cutting wedge working surfaces of the tool with the machined material is the consequence. The formed lubricating films and secondary structure function are integral parts of the tool material and participate in cutting. The formed protective films cannot be exclusively continuous. In addition, their inevitable local abrasion and destruction occur together with the tool material with the formation of chemically pure freshly formed surfaces, which again interact with the external environment in the contact zone at increased heat and power loads (stage 4, Figure 2) and the formation of secondary structures occurs. These stages are cyclically repeated several times throughout machining the part until the cutting tool reaches critical wear.

**Figure 2.** Schematic diagram of the secondary structures' physicochemical formation in the contact zone of the cutting wedge and the workpiece to be machined.

The understanding of physicochemical processes described above for contacting a tool and a workpiece provides researchers with an excellent toolbox for creating selforganizing materials and coatings. It is possible to purposefully design and implement the composition and structure of the tool material and/or the surface coating applied to the tool and predict the formation of certain secondary structures necessary for specific operating conditions, taking into account the thermal effect. It is important to emphasize that obtaining reliable diagnostic information about the changes that have occurred in the ultrathin surface layer of the tool material or coating is possible only by high-precision techniques based on spectroscopy principles. Only by systematically investigating and understanding the nature of the changes and the formation of secondary structures can a scientifically grounded choice of architecture and chemical and phase composition of the tool material and coating be made for high speed and carbide tools and also for ceramic tools and for specific operating conditions to ensure the maximum possible wear resistance.

The basic concept in self-organizing AMCs is based on the elements in their composition, which form oxide phases with weak bonds between atomic planes during heating [52,53]. These phases have good thermal stability and resistance to oxidation at the tribocontact spot with the workpiece being machined (for example, W, Mo, V, and Ti form

a series of oxides with a layered structure). One of the most studied compounds that forms stable oxide phases upon oxidation is vanadium nitride. Since VN is a compound potentially capable of imparting antifriction properties to existing coatings during oxidation, there are solutions for creating superlattice coatings [54], in which layers of nanosized thickness TiAlN/VN alternate. The coefficient of friction of coatings of this type is relatively low at temperatures of 700 ◦C and is no more than 0.5. TiAlN/VN coatings show a lower friction coefficient than hard coatings with superlattice TiAlN/CrN or CrN/NbN [55,56].

The mechanism of formation and the type of secondary structures formed are separate for each coating in their temperature range, and a transition from one joint to another is possible during cutting with an increase in temperature on the contact pads of the tool [57]. For example, the beginning of the formation of oxide phases in the tribocontact zone is noted for a TiAlN/VN coating at 500 ◦C, and a complex structural analysis reveals the presence of vanadium pentoxide V2O5 in the near-surface layer. The specified compound melts with its removal from the surface layer at 685 ◦C, forming high-temperature modifications of other vanadium oxides.

The well-known studies of the mechanism of self-organization in milling with a tool made of WC-Co substrate with multilayer nanosized coatings based on complex nitrides of TiAlCrSiYN/TiAlCrN [16] are very indicative. This coating provides mass transfer of many elements, primarily Al and Si, to the friction surface, accompanied by the formation of dissipative structures due to a high nonequilibrium state and a complex nanocrystalline/layered structure. Ultimately, this leads to a substantial decrease in entropy production and the corresponding wear rate because of self-organization already occurring at the initial stage of wear. The formation of synergistic complex tribooxides endows the surface layer with increased thermal barrier properties. It creates the effect when the intense heat flux accompanying cutting is minimally accumulated in the surface layer of the cutting tool and is maximally transferred to the environment, ensuring the working condition of the tool for a longer time.

The authors of this project comprehensively studied the processes of wear and changes in the structure of the tribocontact zone of the surface of tools from high speed steels and WC-Co substrate and oxide–carbide and oxide–nitride ceramics, including those with TiNbAlN/TiZrN coatings, etc. which were deposited by magnetron sputtering bombarded with fast argon atoms [58–61]. Forming nonstoichiometric amorphous oxide films on the surface of ceramic samples was revealed during the research. Nitride and carbide phases from the surface layer were partially transformed into simple or complex oxides in the contact zone due to tribooxidation. The formation of these secondary structures made it possible to increase the wear resistance of ceramic tools up to two times during turning and milling.

As a typical example of the practical use of the adaptation mechanism in the conditions of frictional and adhesive contact interaction, it will be interesting to consider the results of research of the scientific group of MSTU "STANKIN" devoted to the multilateral study of the structural state and performance properties of improved tool high speed steels.
